1. Field of the Invention
The present invention relates to a camera module in which a lens unit and an image sensor are provided on a resin multilayer board made of a thermoplastic resin and having flexibility.
2. Description of the Related Art
At present, most of portable devices such as cellular phones, PDAs, and the like have a photographing function. As a camera module that realizes such a photographing function, a module including a flexible board is used in some cases (see, e.g., Japanese Unexamined Patent Application Publication No. 2007-165460).
FIG. 6A is a cross-sectional view of a part of an existing camera module 101 disclosed in Japanese Unexamined Patent Application Publication No. 2007-165460, as seen from a side thereof.
The camera module 101 includes a flexible printed circuit board (FPC) 102, an optical unit 103, an image sensor 104, a connector 105, and an infrared (IR) shielding filter 106. The FPC 102 has flexibility. In addition, the FPC 102 has an opening 102A and a plurality of via-hole conductors 102B. The optical unit 103 is provided on one principal surface of the FPC 102 so as to be opposed to the opening 102A. In addition, the optical unit 103 includes a lens barrel 103A and a housing 103B. The housing 103B includes a partition wall which partitions the interior thereof into a space at a side close to the FPC 102 and a space at a side away from the FPC 102. The partition wall has an opening 103C. The lens barrel 103A is provided in the space of the housing 103B at the side away from the FPC 102. The IR shielding filter 106 is provided on the one principal surface of the FPC 102. The IR shielding filter 106 is opposed to the opening 102A within the space of the housing 103B at the side close to the FPC 102. The image sensor 104 is an IC component. In addition, the image sensor 104 is provided on the other principal surface of the FPC 102. The image sensor 104 includes a plurality of photodetectors 104A and a plurality of bumps 104B on a surface thereof opposed to the FPC 102. Each photodetector 104A receives light incident thereon through the lens barrel 103A, the opening 103C of the housing 103B, the IR shielding filter 106, and the opening 102A of the FPC 102 in order. Each bump 104B is connected to the connector 105 via the via-hole conductor 102B of the FPC 102 and a wiring pattern which is not shown.
FIG. 6B is an exploded perspective view showing the FPC 102 and the image sensor 104.
The plurality of photodetectors 104A are arranged in a matrix manner in a rectangular region located at the center of the surface of the image sensor 104 that is opposed to the FPC 102. In addition, the plurality of bumps 104B are arranged in an annular region surrounding the plurality of photodetectors 104A of the image sensor 104.
In the FPC 102, the opening 102A is formed in a region opposed to the plurality of photodetectors 104A of the image sensor 104. In addition, the plurality of via-hole conductors 102B of the FPC 102 are formed in a region opposed to the plurality of bumps 104B of the image sensor 104.
An adhesion method using an anisotropic conductive film (ACF) is used for joining the plurality of bumps 104B of the image sensor 104 and the plurality of via-hole conductors 102B of the FPC 102. It should be noted that in mounting an IC component such as the image sensor 104 on the FPC 102 or the like, an ultrasonic joining method or the like other than an adhesion method may be used.
In a mounting method such as an adhesion method, an ultrasonic joining method, or the like, an IC component such as an image sensor and an FPC may be heated or pressed. If the FPC is formed from a thermoplastic resin, the thermoplastic resin is plasticized even at a relatively low temperature. Thus, in a mounting method involving heating or pressing, flowing of the thermoplastic resin occurs. Meanwhile, flowing of a via-hole conductor formed in the FPC does not occur unless the temperature is sufficiently higher than the temperature at which the thermoplastic resin is plasticized. In addition, the thermoplastic resin less flows around the via-hole conductor. Therefore, in heating or pressing the FPC, the FPC non-uniformly deforms. Thus, the flatness of the FPC may be impaired. For example, a deformation may occur such that a portion at which the via-hole conductor is present projects. If so, the image sensor is mounted on the FPC so as to be tilted. This may have a great effect on the optical characteristics of the camera module. When a plurality of via-hole conductors are non-uniformly arranged or when an electronic component is embedded within an FPC, the tilt of the image sensor tends to be significantly increased.